Our research program is concerned with the dual system of specialized behavioral and neural mechanisms, which evolved in vertebrates as a result of the selective pressures inherent in the food chain. To protect the skin from a predatory attack, distal stimuli detected by the head receptors and peripheral insults from predatory attack converge to integrative mechanisms in dorsal regions of the brain. Learning results in motor avoidance of the peripheral insult elicited by the distal signal. To protect the gut from poisoned food, taste of food in the mouth and visceral feedback from ingested food converge to integrative mechanisms in ventral regions of the brain. Learning results in a food aversion. We study both systems in various species because the utilization of cues and the learning parameters are different within each system. Furthermore, skin defense is a subsystem representative of the behavioral mechanisms for coping with the external environment while gut defense is a subsystem representative of the hedonic mechanisms for regulating the homeostatic environment. The differences between the two systems are relevant to psychobiological theory in general and to learning theory in particular. Learning within the skin defense subsystem is characterized by precise motor movements in time and space. Reliable signals for peripheral insults tend to block weak and redundant signals. On the other hand, learning within the gut system is selective, sluggish and hedonic in nature. Taste, the most reliable cue for poison, has the capacity to potentiate weak distal cues. These data also have practical implications. Food aversions have been used to protect lambs from wild coyotes. Feasibility studies indicate that similar techniques can also protect vegetable crops from raiding baboons, and protect the distinctive eggs of endangered species from crows and ravens. Conditioned flavor aversions have also been used as part of alcohol abuse programs. Conversely, knowlege of conditioned illness parameters can assist the physician to establish chemotherapy treatment schedules to reduce conditioned aversions to nutritious food and assist the clinician to counteract conditioned nausea and vomiting.